1. Field of the Invention
The present invention relates to a laser apparatus and a laser irradiation method, specially a laser apparatus and a laser irradiation method that have a function to stabilize energy of an irradiated laser beam. The present invention also relates to a manufacturing method of a semiconductor device including a step for crystallizing a semiconductor film by the laser apparatus and the laser irradiation method.
2. Description of the Related Arts
Lasers are classified into a gas laser and a solid-state laser according to a laser medium and have various kinds. In the lasers, obtained wavelengths, energy values and pulse characteristics are different depending on the laser mediums. Applications of lasers have been searched according to characteristics of laser beams. Of the various lasers, YAG laser, CO2 laser, excimer laser and the like are mainly used for many industrial laser apparatuses.
An excimer laser of a gas laser emits an intensive ultraviolet beam and has a short wavelength by ArF (0.193 μm) and a short wavelength by XeF (0.351 μm). Further, an excimer laser is superior in light condensing. Therefore the excimer laser is suitable for a field in which a super microfabrication at μm level is required as typified by a semiconductor manufacturing including a mask forming, in addition to general parts processing.
Generally, in a gas laser typified by an excimer laser or a CO2 laser, a pressure fluctuation of a gas that is a laser medium is easily generated in an oscillator, or a fluctuation in gas flow rate is also easily generated in the case that circulating a gas of a laser medium to oscillate in a electric discharge tube. As a result, there is a problem that it is difficult to stabilize energy of an irradiated laser beam from the oscillator and to perform a process to an object evenly.
Conventionally, a pressure fluctuation of a laser medium in a electric discharge tube is detected and another a pressure fluctuation to counteract the fluctuation is generated to suppress the pressure fluctuation or a fluctuation in flow rate of a laser medium, in order to stabilize energy of an irradiated laser beam (Reference 1: Japanese Patent Laid-Open No. Hei 07-038180).
The above reference 1 describes as follows: a pressure fluctuation of a gas that is laser medium is detected in a pressure sensor provided in a gas supplying duct and is input to a feedback control unit as a detection signal. A phase of the detection signal is inverted in the feedback control unit and the detection signal having the inverted phase is subjected to a frequency analysis by fast Fourier transform (FFT) to obtain a waveform signal in a waveform generator. After the waveform signal is amplified, it is input to a wave transducer. The wave transducer generates a pressure fluctuation of the inverted phase with the same amplitude as that of the pressure fluctuation detected by a pressure sensor to counteract the pressure fluctuation of the laser medium.